Bark

From a distance it looked as if this tree had been marked for felling, or some other work, but in fact the orange coloration appears to be naturally occurring, so I assumed it was a bacterial or fungal infection that has got into the place where a small branch has been broken off. I've now looked it up and apparently alders can suffer from phytophthora (fungal) infection, which produces a rusty stain, so that may well be what's happened here. The colour is surprisingly bright in comparison with the healthy bark.